1. Field of the Invention
The present invention relates to a control apparatus for a hybrid vehicle, and particularly relates to a control apparatus for hybrid vehicles, which is capable of recovering the charge-discharge balance by activating the motor during the travel when the charge-discharge balance of the battery device shifts to an over-discharge state.
2. Background Art
Hybrid vehicles provided with a motor in addition to an engine are conventionally known. There are two types of hybrid vehicles: series hybrid vehicles and parallel hybrid vehicles. Series hybrid vehicles are driven by a motor which is driven by a generator driven by the engine.
Sine the motor is not connected mechanically to the engine, the engine can be driven at an approximately constant rotation speed within an speed region wherein the engine can be driven at a lower fuel consumption rate and lower emission rate than the case of a conventional engine.
In contrast, in parallel hybrid vehicles, a motor directly connected to the engine assists the engine in rotating the drive shaft, and the motor is used as a generator for charging electric energy into a battery device, and the electric energy generated by the motor is used for a variety of electric equipment in the hybrid vehicle.
Thus, in parallel hybrid vehicles, since the driving load of the engine can be reduced, the fuel consumption rate and the emission rate can be improved.
There are several types of the above-described parallel hybrid vehicle, one is a type in which the motor for assisting the output of the engine is directly connected to the engine axis, and the motor functions as a generator for charging the battery when the hybrid vehicle decelerates, and the other one is a type in which both or either one of the engine and the motor can generates the driving force and a generator is additionally provided.
In hybrid vehicles such as those described above, the demands of the driver can be satisfied by preserving the electric energy of the battery (hereinafter, called “state of charge”, that is “remaining battery charge”) by carrying out a variety of controls so that the motor assists the engine at the time of acceleration, and the motor charges the battery by deceleration regeneration at the time of deceleration. For example, since a large amount of deceleration regeneration can be obtained after high speed travel, a part of the energy drawn from the battery can be recovered at the time of deceleration. That is, after climbing the uphill, the battery can be charged while travelling downhill (this technique is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 7-123509).
However, in conventional hybrid vehicles, when driven the hybrid vehicle without sufficient deceleration acceleration, the state of charge gradually decreases while driving the vehicle. Therefore, the problem arises that, when it is necessary to drive the car uphill, and subsequently drive on a flat road, the battery charge consumed at the time of driving the uphill can not be restored because no downhill regeneration is provided.